This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Optical Power Divider And Fabrication Method Thereof earlier filed in the Korean Industrial Property Office on Nov. 5, 1998, and there duly assigned Serial No. 97-58240 by that Office.
1. Field of the Invention
The present invention relates to an optical power divider, and more particularly, to an optical power divider using a beam separator and a beam expander, and a method for fabricating the same.
2. Description of the Related Art
In general, an optical power divider is for distributing incident light among a plurality of regions. The optical power divider can be applied to various fields such as an optical communications system or an optical access network, and is a basic element used for optical communications. The optical power divider may be classified according to the number of output ports, into 1xc3x972, 1xc3x974, 1xc3x978, . . . , 1xc3x97N types, where N=2m and m is a natural number. The light is usually output to each output port with a uniform ratio. However, a splitting ratio at each port may be different for a special application. As shown in FIGS. 1A and 1B, the optical power divider is expanded by connecting Y-type branched optical waveguides each having two output ports in series and parallel to each other, like a tree. That is, the 1xc3x974 type optical power divider of FIG. 1A is formed by connecting three 1xc3x972 Y-type branched optical waveguides in series and parallel to each other, and the 1xc3x978 type optical power divider of FIG. 1B is obtained by connecting seven 1xc3x972 Y-type branched optical waveguides in series and parallel to each other.
Assuming that the optical power divider is expanded by the above manner, more serial connections are required as the number of output ports increases. Thus, the length of a 1xc3x974 branched optical waveguide or a 1xc3x978 branched optical waveguide is two or three times the length of the 1xc3x972 branched optical waveguide. In addition, as the length of an optical power divider increases, propagation loss of the light also increases. Here, we have determined that the length of the Y-type branched optical waveguide can be reduced by increasing a branch angle within the structure of the optical waveguide. We have also determined, however, that such an increase in the branch angle increases radiation loss at a branch area, so that decreasing the length of the branched optical waveguide has limitations. Thus, in order to increase the branch angle while maintaining the radiation loss at a minimum, a branch area has been designed to have a specific structure [H. P. Chan and P. S. Chung, Electron Lett., vol. 32, pp. 652-654, 1996], and a microprism structure has been inserted in a branch area [H. B. Lin, R. S. Cheng and W. S. Wang, IEEE Photon. Technol. Lett., vol. 6, pp. 825-827, 1994].
To solve the above problems, it is an object of the present invention to provide an optical power divider adopting a beam separator and beam expanders such that additional loss does not occur and such that the length of the optical power divider does not increase when output ports of the optical power divider are expanded.
It is another object of the present invention to provide a method for fabricating the optical power divider.
According to an aspect of the first object, there is provided an optical power divider comprising: an input optical waveguide having an input port for receiving incident light, for guiding the light incident via the input port; a plurality of output optical waveguides having at least two output ports, for outputting the light incident via the input optical waveguide to the output ports, wherein the number of output optical waveguides is equal to one less than the number of the output ports; and a beam separator located at a branch area in which the light incident on the input optical waveguide diverges toward the output optical waveguides, the beam separator being made of a material having a refractive index lower than the core of the input and output optical waveguides, for separating the light to the output optical waveguides with a predetermined ratio.
Preferably, the optical power divider further comprises beam expanders located near the outer sides of the branch area in which the light incident on the input optical waveguide diverges toward the output optical waveguides, the beam expander being made of a material having a refractive index higher than the cladding region of the input and output optical waveguides, for dividing the light to be output uniformly to the output ports.
Preferably, the beam separator has a triangular shape, and the light output to the output ports is separated in a predetermined ratio according to the refractive index, and the height and the length of a side of the triangle.
According to another aspect of the first object, there is provided an optical power divider comprising: an input optical waveguide having an input port for receiving incident light, for guiding the light incident via the input port; a plurality of output optical waveguides having at least two output ports, for outputting the light incident via the input optical waveguide to the output ports, wherein the number of output optical waveguides is equal to that of the output ports, and beam expanders located near the outer sides of the branch area in which the light incident on the input optical waveguide diverges toward the output optical waveguides, the beam expander being made of a material having a refractive index higher than the cladding region of the input and output optical waveguides, for dividing the light to be output uniformly to the output ports.
Preferably, the beam expander has a triangular shape, and the light output to the output ports is separated in a predetermined ratio according to the refractive index, and the height and the length of a side of the triangle.
According to still another aspect of the first object, there is provided an 1xc3x974 optical power divider comprising: an input optical waveguide having an input port for receiving incident light, for guiding the light incident via the input port; four output optical waveguides having four output ports, for outputting the light incident via the input optical waveguide to the output ports; a beam separator located at the point of symmetry between upper and lower regions of the branch area in which the light incident on the input optical waveguide diverges toward the four output optical waveguides, with a triangular shape, the beam separator being made of a material having a refractive index lower than the core of the input and output optical waveguides, for separating the light to the output optical waveguides with a predetermined ratio according to the refractive index, and the height and the length of a side of the triangle; and beam expanders located near the outer sides of the branch area in which the light incident on the input optical waveguide diverges toward the output optical waveguides, the beam expander being made of a material having a refractive index higher than the cladding region of the input and output optical waveguides, for dividing the light to be output uniformly to the output ports according to the refractive index, and the height and the length of a side of the triangle.
Preferably, the refractive index of the beam separator is the same as that of the cladding region of the input and output optical waveguides, and the refractive index of the beam expander is the same as that of the core of the input and output optical waveguides.
Preferably, branch angle between the four output optical waveguides are the same.
Preferably, assuming that the four output optical waveguides are called first, second, third and fourth output optical waveguides from the top, and there are an imaginary line AAxe2x80x2 parallel to the width of the input optical waveguide and passing through a crossing point b between the inner sides of the first and fourth output optical waveguides, and an imaginary line BBxe2x80x2 parallel to the width of the input optical waveguide and passing through a crossing point c between the inner sides of the second and third output optical waveguides, the beam separator is located at the center of a branch area between the imaginary lines AAxe2x80x2 and BBxe2x80x2 and the beam separator has an isosceles-triangular shape having the point c as the center point of the base and the point b as the crossing point between the legs of the triangle with equal length.
Preferably, the beam expanders are located near the outer sides of the first and fourth output optical waveguides between the imaginary lines AAxe2x80x2 and BBxe2x80x2, with a triangular shape, a side of the triangle is a part of the imaginary line BBxe2x80x2, and a crossing point of the triangle is located on the imaginary line AAxe2x80x2.
Preferably, assuming that the four output optical waveguides are called first, second, third and fourth output optical waveguides from the top, and there are an imaginary line AAxe2x80x2 parallel to the width of the input optical waveguide and passing through a crossing point b between the inner sides of the first and fourth output optical waveguides, and an imaginary line BBxe2x80x2 parallel to the width of the input optical waveguide and passing through a crossing point c between the inner sides of the second and third output optical waveguides, the beam separator is located at the symmetry center between upper and lower region of a branch area between the imaginary lines AAxe2x80x2 and BBxe2x80x2 and the beam separator has an isosceles-triangular shape having the point b as the crossing point between the legs of the triangle with equal length, and the height greater than the interval between the imaginary lines AAxe2x80x2 and BBxe2x80x2.
Preferably, the beam expanders are located near the outer sides of the first and fourth output optical waveguides between the imaginary lines AAxe2x80x2 and BBxe2x80x2, with a triangular shape, a crossing point of the triangle is located on the imaginary line AAxe2x80x2, and the height of the triangle is greater than the interval between the imaginary lines AAxe2x80x2 and BBxe2x80x2.
Preferably, the beam separator is located at the center of the branch area between the imaginary lines AAxe2x80x2 and BBxe2x80x2, has an isosceles-triangular shape having the point c as the center of the base and the point b as the crossing point between the legs of the triangle with equal length, and the interval between the second and third output optical waveguides on the imaginary line BBxe2x80x2 is equal to the length of the base of the triangular beam separator.
According to an aspect of the second object, there is provided a method for fabricating an optical power divider including an input optical waveguide for guiding an incident light, at least two output optical waveguides for guiding the incident light passed through the input optical waveguide, a beam separator for separating the incident light, the beam separator being located at a branch area in which the incident light via the input optical waveguide diverges to the output optical waveguides and made of a material having a refractive index lower than that of the core of the input and output optical waveguides, and beam expanders for expanding the incident light, the beam expanders being located near the outer sides of the branch area in which the incident light via the input optical waveguide diverges to the output optical waveguides and made of a material having a refractive index higher than that of the cladding region of the input and output optical waveguides, the method comprising the steps of: (a) forming a thin film as a lower clad, on a substrate; (b) growing a thin film as a core, on the thin film formed in step (a), having a refractive index higher than the thin film of step (a); (c) selectively etching the resultant of step (b) to form the structure of the optical power divider; and (d) growing an upper clad on the resultant of step (c).
Preferably, the substrate is formed of a material selected from the group consisting of Si, GaAs and InP, and the lower clad, the core and the upper clad are formed as semiconductor thin films.
Preferably, the substrate is formed of silicon or fused silica, and the lower clad, the core and the upper clad are formed of silica or polymer.
According to another aspect of the second object, there is provided a method for fabricating an optical power divider, comprising the steps of: (a) forming a core layer on a ferroelectric substrate, having a refractive index higher than the ferroelectric; (b) etching the resultant of the step (a) to form the structure of the optical power divider; and (c) forming an upper cladding layer on the optical waveguide formed in the step (b).
According to still another aspect of the second object, there is provided a method for fabricating an optical power divider, comprising the steps of: (a) forming an optical waveguide on a ferroelectric substrate, by increasing a refractive index of a predetermined region in the structure of the ferroelectric substrate; and (b) forming an upper clad layer on the optical waveguide.
Preferably, the ferroelectric is LiNbO3 or LiTaO3.
Preferably, the refractive index is increased by a proton exchange method for substituting hydrogen ions for lithium ions within the ferroelectric substrate, or by in-diffusing a metal thin film.
Preferably, the metal thin film is a titanium thin film or a nickel thin film, and the upper clad layer is a silica thin film or an alumina thin film.